Modern combat aircraft require the flexibility of being able to fly at either subsonic or supersonic speeds and also must be highly maneuverable. Particularly at supersonic speed it is desirable to have the exhaust nozzle in the form of a convergent/divergent nozzle. Furthermore for afterburning it is required that the nozzle be operable to increase the throat area.
Vectoring of the nozzle is also desirable to achieve yaw and pitch control of the aircraft. This is accomplished by manipulating the divergent portion of the nozzle to direct the exhaust gases an angle with the axis of flow through the nozzle.
Axisymmetric nozzles have tended to have in the order of fifteen to eighteen trains of flaps, and also fifteen to eighteen trains of seals. These nozzles have not been readily adaptable to use a lesser number which would achieve a lesser weight nozzle. With fewer flaps the circumferential loads being transferred around the periphery must be capable of passing around sharper corners. Also pressure loaded seals have been used between adjacent flaps. These depend on the internal pressure to maintain the seals in contact with the flaps. At the extreme angles between adjacent flaps existing with fewer flaps, the pressure loading is too extreme for dependable sliding seals.